Disconnect the negative post. Get a battery charger and start charging. It will take time as in many hours to charge if its really dead. A charged battery is 12.6V DC. Get a voltmeter and measure the voltage on the battery. Turn on the lights for a few minutes before measuring. What's the voltage?
If it starts, what's the voltage on the battery w/ the engine running?
If you suspect the battery is being drained, keep the negative disconnected when not in use.
Check w/ a test light between the negative post and cable.
Or that's what I'd do...
